Dmitri Mendeleev was a Russian chemist who lived in the mid 1800s. He was interested in the patterns he observed in the elements’ chemical properties (how they react) and physical properties (e.g., colour, density, melting and boiling points) as the atomic mass increased.
He made a card for each of the 63 elements known at the time and, after much rearranging, found a way to order them according to the patterns he observed. There were gaps in his table and he predicted these spaces belonged to elements which existed but had yet to be discovered. Mendeleev’s periodic table, that he published in 1869, is the periodic table that we still use today.
Today we know more about the subatomic structure (protons, neutrons, and electrons) of the elements and the patterns have become a lot clearer.